This article provides guidelines for selecting pressure sensing elements used in process plants. Main keywords for this article are Pressure Sensing Elements Selection Guide. Pressure Sensing Elements Definitions. Bourdon Tube Selection Guide. Bellows Selection Guide.
Pressure Sensing Elements Definitions
Accuracy.
The degree to which an indicated value matches the actual value of a measured variable.
Bourdon Tube.
A pressure sensing element consisting of a twisted or curved tube of noncircular cross section which tends to be straightened by the application of internal pressure.
Direct Drive.
Any powered mechanism where the driven portion is on the same shaft as the driving portion, or is coupled directly to the directly to the driving portion.
Gradient.
The rate of change of some variable with respect to another, especially a regular uniform or stepwise rate of change.
Hysteresis.
The tendency of an instrument to give a different output for a given input, depending on whether the input resulted from an increase or decrease from the previous value.
Manometer.
A gage for measuring pressure or a pressure difference between two fluid chambers. A U-tube manometer consists of two legs, each containing a liquid of known specific gravity.
Overrange.
In process instrumentation, of a system or element, any excess value of the input signal above its upper range value or below its lower range value.
Precision.
The value of the smallest incremental difference that can be measured by a given instrument or measurement system.
Pressure Element.
The portions of a pressure measuring gage that move or are temporarily deformed by the system pressure, the amount of movement or deformation being proportional to the pressure.
Reliability.
The probability that a device will perform its objective adequately, for the period of time specified, under the operating conditions specified.
Repeatability.
The closeness of agreement among a number of consecutive measurements of the output for the same value of the input under the same operating conditions, approaching from the same direction,
for full-range traverses.
Semiconductor.
Materials, used for sensing elements or transduction elements, whose resistivity falls between that of conductors and insulators.
Sensing Element.
That part of the transducer which responds directly to the measurand.
Sensitivity.
The ratio of the change in output magnitude to the change of the input which causes it after the steady-state has been reached.
Span.
The algebraic difference between the upper and lower range values.
Strain Gage.
A high resistance, fine wire or thin foil grid for use in a measuring bridge circuit. When the grid is securely bonded to a specimen, it will change its resistance as the specimen is stressed. These devices are used in many forms of transducers.
Transmitter.
A device that senses a process variable through the medium of a sensor and has an output whose steady-state value varies only as a predetermined function of the process variable.
Pressure Sensing Elements
The following characteristics for various pressure sensing elements shall be properly understood before selecting from Table I.
Bourdon Tube Selection Guide
- Bourdon tubes, also known as elastic deformation elements, operate on the following principle: When the pressure is applied to an open end of a bend tube, it straightens out, and when pressure is
removed the elasticity of metal returns the tube to its original position. - Bourdon tubes are widely used for local indication, signal transmission to remote locations, and as receiver instruments. Bourdon tubes are of 3 types: C-Tube, Spiral, and Helical.
a. C-Tube
(i) C-Tubes are used in most pressure gages and pressure transmitters. They are not used in receiver instruments.
(ii) C-Tubes are accurate, rugged, reliable, and inexpensive.
(iii) C-Tubes shall be provided with overrange stops to protect them against damage due to over pressure.
(iv) Operating range of a C-Tube element is from 15 psig to 100,000 psig.
(v) Accuracy of ±0.1 to 2.0 percent is recommended. Accuracy of ± 1.0 percent is considered normal by the industry.
b. Spiral and Helical Pressure Elements
(i) These are used primarily in pressure transmitters and receiver instruments. Their use in pressure gages is limited.
(ii) They are more capable to be used as direct drive read-out devices. They are often used for low pressure applications where higher accuracy and repeatability is required as compared to C-Tube
design.
(iii) Accuracy of ± 0.5 percent is recommended.
(iv) Spiral elements cover the range from vacuum to as high as 4,000 psi. Helical elements cover the range from 200 to 80,000 psi. - Advantages of Bourdon tube: low cost, simple construction, proven in the industry, high pressure ranges, good accuracy versus cost except at low ranges, improved design at high pressures for maximum
safety. - Limitations of Bourdon tube: low spring gradient, limited precision measurement at pressures below 50 psi, susceptible to shock and vibration due to their large overhang, subjected to hysteresis.
Bellows Selection Guide
- This type of element is sensitive, accurate and is used primarily for lower pressure applications found in Receiver Instruments.
- The accuracy of Bellow element is ± 0.5 percent of the span. It can be used for the measurement in the range of vacuum to 4,000 psi.
Diaphragm Selection Guide
- Diaphragm elements are sensitive to very low pressure and can be used to measure full range pressure values from 5 to 10 inches of water column.
- Diaphragm elements are used in pressure gages and pressure transmitters.
- Accuracy of ± 0.1 to 2.0 percent is recommended.
Strain Gage Selection Guide
- The strain gage is based on the principle of change of electrical resistance of a wire under strain.
- Metal and metal alloys in wire and foil forms are used for standard strain gages. Semiconductor strain gages are also available.
- Accuracy of ±0.1 to 2.0 percent is recommended.
- They are available in a range of a few to 200,000 psi.
Manometers Selection Guide
- Manometers can provide very accurate means of pressure measurement. They are used as secondary standards for calibration of shop instruments.
- The measured pressure range of most of manometers is between a few to 30 psi. It depends on physical length of the tubes and the specific gravity of filled fluid.
- The tubes for manometers used as laboratory standards shall be greater than 0.375 inch in diameter.
- While water and mercury are the most common liquids used in manometers, other fluids are used in industrial applications.
- To measure low pressures, an inclined tube manometers is used.
Other Types of Sensors
Other types of sensors for pressure measurements are: Capacitative, Resonant wire, and Piezoelectric. Their application range and accuracy is given in Table I.
Pressure Sensing Elements Selection Guide
In addition to the following guidelines, cost, proven design, material of construction, and mounting shall also be considered in the selection of pressure elements.
- For direct pressure measurement in the range of 15 psig to 100,000 psig and an accuracy requirement of ± 1.0 percent, the Bourdon element shall be used.
- For direct pressure measurement in the range of vacuum to 40 psig and an accuracy requirement of ± 0.5 percent, the Bellow element shall be used.
- For Direct Pressure Measurement in the range of vacuum to 200 psig and an accuracy requirement of ± 1.0 percent, the Diaphragm element shall be used.
- Pressure sensors are used in transmitters, controllers, and switches. Sensors for these devices shall be selected considering specific applications.
- While selecting pressure sensing element from Table I , consideration shall also be given to the following factors:
(i) Material of construction
(ii) Mounting
(iii) Type of transmission
(iv) Vendor support
(v) Past performance and use of device in a similar industry